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Comparison of the thermal cycling performance of thermal barrier coatings under isothermal and heat flux conditions.

Fry, A T; Banks, J P; Nunn, J; Brown, L J (2008) Comparison of the thermal cycling performance of thermal barrier coatings under isothermal and heat flux conditions. Mater. Sci. Forum, 595-59. pp. 77-85.

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Ceramic Thermal Barrier Coatings (TBCs) have been developed for advanced gas turbine engine components to improve the engine efficiency and reliability. The TBCs act to reduce the mean metal temperature of the coated component thereby allowing designers to extend the temperature range in which a metal can be used. The integrity and reliability of these coatings is therefore of paramount importance. Accurate prediction of service lifetimes for these components relies upon many factors, and is not straightforward as knowledge of the service conditions and accurate input data for modelling are required. TBCs fail in service by two main mechanism, spallation and/or erosion. For the purposes of this study only failure by spallation will be considered.

Spallation of TBCs occurs when the thermally grown oxide (TGO) that forms between the TBC and the bondcoat beneath it loses adhesion. The main cause of this debonding is the development of thermally induced strains between the metallic bondcoat and the alumina TGO layers developed due to the thermal expansion. Thermal transients due to the power cycles of turbines will then cause these fractures to grow between the TGO and the bondcoat as shown in Figure 1. When these fractures reach a critical size they can grow rapidly and cause the TBC to spall off. Thermal cycling of TBCs is used therefore to evaluate and rank TBC performance. Within the laboratory this is often conducted under isothermal conditions, whereby a test specimen is alternately placed in a furnace and removed from the furnace to cool either naturally or with the addition of forced cooling, usually via blasting with compressed air. Whilst this test method has performed adequately in the past it does not fully simulate service conditions. Work has been underway therefore to develop a more complex test method, which better simulates the service conditions experienced by the TBC. The approach used by NPL and other organisations uses a gas torch to heat the operating face of the TBC whilst cooling the rear of the substrate with compressed air, thereby imparting a heat flux on the specimen. The specimen is then cycled by removing the gas torch and cooling with compressed air on the front and rear faces. Using this method, temperature differences on steel discs without a TBC of around 200 °C have been achieved, giving a conductive heat flux across the specimen width of around 4000 kWm-2. The test rig developed at NPL for this purpose is shown in Figure 2. Tests have been conducted on a number of TBC systems, the results in this work concentrates on two systems consisting of an IN738 substrate with two different bondcoats (997 VPS and CN334) with an EBPVD TBC. Thermal cycling tests have been performed under both isothermal and heat flux conditions. During the course of the tests the samples were examined non-destructively using a thermal camera to identify early indications of spallation. Once spallation had been identified the samples were removed from testing and metallographic examination performed.

This paper reports on the performance of the flame rig equipment and the results of the comparison of the two test methods for the TBC system described above.

Item Type: Article
Keywords: TBC, thermal cycling, heat flux
Subjects: Advanced Materials
Advanced Materials > Metals and Alloys
Last Modified: 02 Feb 2018 13:15
URI: http://eprintspublications.npl.co.uk/id/eprint/4341

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